Method and system for engine control

ABSTRACT

Methods and systems are provided for integrating a VCR engine with a CVT transmission. Responsive to a driver demand, a controller may determine whether to maintain a current compression ratio or transition to an alternate compression ratio based on the fuel economy benefit of the transition and further based on any engine limitations that may be incurred at the engine speed-load following the transition. To improve the net fuel economy benefit while addressing the engine limitation, a compression ratio transition may be combined with a CVT adjusted engine speed-load regime, while maintaining engine power output.

FIELD

The present description relates generally to methods and systems forcontrolling an engine compression ratio in a hybrid electric vehiclesystem.

BACKGROUND/SUMMARY

The compression ratio of an internal combustion engine is defined as theratio of the cylinder volume when the piston is at bottom-dead-center(BDC) to the cylinder volume when the piston is at top-dead-center(TDC). In general, the higher the compression ratio, the higher thethermal efficiency of the internal combustion engine. This in turnresults in improved fuel economy and a higher ratio of output energyversus input energy of the engine. In conventional engines, thecompression ratio is fixed and thus the engine efficiency cannot beoptimized during operating conditions to improve fuel economy and enginepower performance.

Various technologies have been developed to enable the compression ratioof an engine to be varied with engine operating conditions. One exampleapproach is shown by Yoshida et al. in U.S. Pat. No. 7,258,099. Therein,cam timing adjustments are used to vary the effective compression ratio.For example, a late intake valve closing is used to reduce the effectivecompression ratio. Still other approaches, such as shown by Kamada etal. in US20130055990, rely on a piston displacement changing mechanismthat moves the pistons closer to or further from the cylinder head,thereby changing the size of the combustion chambers.

However the inventors herein have recognized potential issues with suchapproaches. As one example, the optimal fuel economy gain associatedwith adjusting a compression ratio may not be realized due to the fixedgear ratio of the transmission. In particular, at a given driver demand,for each compression ratio of the engine, there may be an associatedfixed engine speed and load range that meets the driver demand. Anengine controller may transition to a more fuel efficient compressionratio for the driver demand. However, upon changing compression ratios,there may be engine limitations experienced at the associated enginespeed-load that may reduce the fuel economy benefit of the compressionratio transition. As an example, upon transitioning to a highercompression ratio, the engine may become more knock-limited at highloads. The fuel penalty associated with the knock mitigation mayoutweigh the fuel economy benefit of the compression ratio transition.As another example, upon transitioning to a lower compression ratio, theengine may become more friction limited at low loads. Another issue isthat frequent changes in operator pedal demand may cause the engine loadto move back and forth, leading to frequent switching betweencompression ratios. Excessive compression ratio switches can degradefuel economy due to losses incurred during transitions.

The inventors herein have recognized that the fuel economy benefits of avariable compression ratio (VCR) engine may be better leveraged throughintegration with a continuously variable transmission (CVT). Inparticular, the CVT may enable the engine speed and load to be adjustedwhile maintaining the fuel efficient compression ratio and whilemaintaining the power output of the engine. In one example, fuel economymay be improved by a method for an engine coupled to a CVT comprising,for a desired power level, comparing engine efficiency at a currentcompression ratio to engine efficiency at a modified compression ratiowith an adjusted engine speed-load; and in response to a higher thanthreshold improvement in the engine efficiency at the modifiedcompression ratio with the adjusted engine speed-load, transitioning tothe modified compression ratio and adjusting to the adjusted enginespeed-load. In this way, an engine can be operated with a compressionratio that provides an improved fuel economy for a given driver demandwithout being excessively knock limited at higher loads. In addition,the need for frequent compression ratio switching can be reduced.

As an example, an engine system may be configured with a VCR enginecoupled to vehicle wheels via a CVT transmission. The VCR engine may beconfigured with a piston position changing mechanism that enables thecompression ratio (CR) to be varied between at least a lower value and ahigher value. For a given driver demanded power level, an enginecontroller may compare the fuel efficiency for each of the higher CR andthe lower CR. Then, for the more fuel efficient compression ratio, thecontroller may predict if there are any limitations, such as knocklimitations, associated with the corresponding engine speed-load. If so,the controller may further determine if the engine speed-load can bechanged while maintaining the selected CR and while maintaining thedemanded engine power output, and any fuel penalties associatedtherewith. If the engine speed-load can be changed after transitioningthe CR with a net fuel economy improvement, the controller may proceedwith the CR transition. Else, the original CR may be maintained. As anexample, upon transitioning to a higher compression ratio, for a givendriver demand, the engine speed may increase while the engine loaddecreases. To address knock anticipated at the higher compression ratio,an engine controller may actuate the CVT to increase the engine speedwhile decreasing the engine load so as to maintain the demanded enginepower output while providing a net fuel benefit. Likewise, whentransitioning to a lower compression ratio, the engine speed may belowered (from the previous engine speed for the higher CR) while load isincreased (as compared to the previous load for the higher CR).

In this way, fuel economy benefits can be improved. The technical effectof integrating VCR engine technology in a vehicle having a CVTtransmission is that for a given driver demanded power, the benefits ofa variable compression ratio can be better leveraged. In particular, theengine speed and torque for a given driver demanded power can beadjusted to reduce knock limitations at higher loads and friction lossesat lower loads, while accounting for changes in compression ratio. Thetechnical effect of assessing the fuel economy benefit of changing thecompression ratio with the fuel penalty associated with operating at theengine speed-load profile corresponding to the selected compressionratio is that frequent CR switching can be reduced. In addition, engineoperation in a more fuel efficient compression ratio can be extendeddespite changes in driver or wheel power demand.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example vehicle powertrain.

FIG. 2 shows a partial engine view.

FIG. 3 shows a high level flow chart for selecting an engine compressionratio in a VCR engine and adjusting an engine speed-load profile in theselected CR with a continuously variable transmission.

FIG. 4 shows an example map for selecting compression ratio usage.

FIG. 5 shows example VCR and CVT adjustments during engine operation.

FIG. 6 shows example BSFC maps for an engine at two differentcompression ratios.

DETAILED DESCRIPTION

The following description relates to systems and methods for improvingfuel economy in a vehicle having a continuously variable transmission(herein also referred to as a CVT), such as the powertrain of FIG. 1.The powertrain may include an engine configured with a piston whoseposition within a combustion chamber can be varied, as described withreference to the engine system of FIG. 2. A controller may be configuredto perform a control routine, such as the example routine of FIG. 3, toselect a compression ratio while adjusting an engine speed-load profileat the selected CR via adjustments to a speed ratio of the CVT to betterleverage the fuel economy benefits of the VCR engine. The controller maycompare fuel island data maps for each compression ratio, such as themaps of FIG. 6. An example map that may be used by the controller toselect whether to maintain or transition compression ratios is shownwith reference to FIG. 4. An example engine operation with CR and CVTadjustments is shown at FIG. 5. In this way, VCT technology can beintegrated and synergized with CVT technology to achieve significantfuel economy improvements.

Referring to FIG. 1, internal combustion engine 10, further describedherein with particular reference to FIG. 2, is shown coupled to torqueconverter 11 via crankshaft 40. Torque converter 11 is also coupled totransmission 15 via turbine shaft 17. In one embodiment, transmission 15comprises an electronically controlled transmission with a plurality ofselectable speed ratios. Transmission 15 may also comprises variousother gears, such as, for example, a final drive ratio (not shown). Inthe depicted example, transmission 15 is a continuously variabletransmission (CVT). The CVT may be an automatic transmission that canchange seamlessly through a continuous range of effective speed ratios,in contrast with other mechanical transmissions that offer a finitenumber of fixed gear ratios (speed ratios). The speed ratio flexibilityof the CVT allows the input shaft to maintain a more optimized angularvelocity. As elaborated with reference to FIGS. 3-4, by adjusting aspeed ratio of the CVT, an engine controller may be configured to varyan engine speed-load profile while maintaining a demanded power outputof the engine. For example, an engine speed may be lowered while anengine load is correspondingly increased to maintain a power output byadjusting the CVT to a lower speed ratio. As another example, an enginespeed may be raised while an engine load is correspondingly decreased tomaintain a power output by adjusting the CVT to a higher speed ratio.This enables fuel economy benefits of operating an engine in a selectedcompression ratio to be better leveraged.

Torque converter 11 has a bypass clutch (not shown) which can beengaged, disengaged, or partially engaged. When the clutch is eitherdisengaged or being disengaged, the torque converter is said to be in anunlocked state. Turbine shaft 17 is also known as a transmission inputshaft.

Transmission 15 may further be coupled to tire 19 via axle 21. Tire 19interfaces the vehicle (not shown) to the road 23. Note that in oneexample embodiment, this power-train is coupled in a passenger vehiclethat travels on the road. While various vehicle configurations may beused, in one example, the engine is the sole motive power source, andthus the vehicle is not a hybrid-electric, hybrid-plug-in, etc. In otherembodiments, the method may be incorporated into a hybrid vehicle.

FIG. 2 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10, such as engine 10 of FIG. 1. Engine 10may receive control parameters from a control system includingcontroller 12 and input from a vehicle operator 130 via an input device132. In this example, input device 132 includes an accelerator pedal anda pedal position sensor 134 for generating a proportional pedal positionsignal PP. Cylinder (herein also “combustion chamber”) 14 of engine 10may include combustion chamber walls 136 with piston 138 positionedtherein. Piston 138 may be coupled to crankshaft 140 so thatreciprocating motion of the piston is translated into rotational motionof the crankshaft. Crankshaft 140 may be coupled to at least one drivewheel of the passenger vehicle via a transmission system. Further, astarter motor may be coupled to crankshaft 140 via a flywheel to enablea starting operation of engine 10.

Engine 10 may be configured as a variable compression ratio (VCR) enginewherein the compression ratio (CR) of each cylinder (that is, the ratioof the cylinder volume when the piston is at bottom-dead-center (BDC) tothe cylinder volume when the piston is at top-dead-center (TDC)) can bemechanically altered. The CR of the engine may be varied via a VCRactuator 202 actuating a VCR mechanism 204. In some example embodiments,the CR may be varied between a first, lower CR (wherein the ratio ofcylinder volume when the piston is at BDC to the cylinder volume whenthe piston is at TDC is smaller) and a second, higher CR (wherein theratio is higher). In still other example, embodiments, there may bepredefined number of stepped compression ratios. Further still, the CRmay be continuously variable between the first, lower CR and the second,higher CR (to any CR in between).

In one example, VCR mechanism 204 is coupled to piston 138. Therein, theCR of the engine may be varied via a VCR mechanism that changes a pistonTDC position. For example, piston 138 may be coupled to crankshaft 140via a piston position changing VCR mechanism that moves the pistonscloser to or further from the cylinder head, thus changing the size ofcombustion chamber 14. In one example, changing the position of thepiston within the combustion chamber also changes the relativedisplacement of the piston within the cylinder. The piston positionchanging VCR mechanism may be coupled to a conventional cranktrain or anunconventional cranktrain. Non-limiting example of an unconventionalcranktrain to which the VCR mechanism may be coupled include variabledistance head crankshafts and variable kinematic length crankshafts. Inone example, crankshaft 140 may be configured as an eccentric shaft. Inanother example, an eccentric may be coupled to, or in the area of apiston pin, the eccentric changing the position of the piston within thecombustion chamber. Movement of the eccentric may be controlled by oilpassages in the piston rod.

It will be appreciated that still other VCR mechanisms that mechanicallyalter the compression ratio may be used. For example, the CR of theengine may be varied via a VCR mechanism that changes a cylinder headvolume (that is, the clearance volume in the cylinder head). It will beappreciated that as used herein, the VCR engine may be configured toadjust the CR of the engine via mechanical adjustments that vary apiston position or a cylinder head volume. As such, VCR mechanisms donot include CR adjustments achieved via adjustments to a valve or camtiming.

By adjusting the position of the piston within the cylinder, aneffective (static) compression ratio of the engine (that is a differencebetween cylinder volumes at TDC relative to BDC) can be varied. In oneexample, reducing the compression ratio includes reducing a displacementof the piston within the combustion chamber by increasing a distancebetween a top of the piston from a cylinder head. For example, theengine may be operated at a first, lower compression ratio by thecontroller sending a signal to actuate the VCR mechanism to a firstposition where the piston has a smaller effective displacement withinthe combustion chamber. As another example, the engine may be operatedat a second, higher compression ratio by the controller sending a signalto actuate the VCR mechanism to a second position where the piston has alarger effective displacement within the combustion chamber. Aselaborated herein, changes in the engine compression ratio may beadvantageously used to improve fuel economy. In addition, bycoordinating CVT adjustments with CR adjustments (FIG. 3), the enginemay be operated with a modified engine speed-load profile in theselected CR, so that synergistic improvements in fuel economy areattained. The modified engine speed-load profile may be modified from adefault speed-load profile of the selected CR to account for enginelimitations at the default speed-load profile, such as knock limitationsor frictional losses. As such, the CVT adjustment enables the enginespeed-load profile to be modified such that the engine power level atthe selected CR in the modified speed-load profile is the same as thepower level in the default speed-load profile (FIG. 4). As used herein,the power level corresponds to a powertrain output of the engine whichis determined as a product of engine load and engine speed.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 2 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor of the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 2, or alternatively may be providedupstream of compressor 174.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The position ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors 155 and 157, respectively. In alternative embodiments,the intake and/or exhaust valve may be controlled by electric valveactuation. For example, cylinder 14 may alternatively include an intakevalve controlled via electric valve actuation and an exhaust valvecontrolled via cam actuation including CPS and/or VCT systems. In stillother embodiments, the intake and exhaust valves may be controlled by acommon valve actuator or actuation system, or a variable valve timingactuator or actuation system.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock. The compression ratio may also be variedbased on driver demand via adjustments to a VCR actuator 202 thatactuates a VCR mechanism 204, varying the effective position of piston138 within combustion chamber 14.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including one fuel injector 166. Fuelinjector 166 is shown coupled directly to cylinder 14 for injecting fueldirectly therein in proportion to the pulse width of signal FPW receivedfrom controller 12 via electronic driver 168. In this manner, fuelinjector 166 provides what is known as direct injection (hereafter alsoreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 2shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may improve mixing and combustion when operating the enginewith an alcohol-based fuel due to the lower volatility of somealcohol-based fuels. Alternatively, the injector may be located overheadand near the intake valve to improve mixing. Fuel may be delivered tofuel injector 166 from a high pressure fuel system 8 including fueltanks, fuel pumps, and a fuel rail. Alternatively, fuel may be deliveredby a single stage fuel pump at lower pressure, in which case the timingof the direct fuel injection may be more limited during the compressionstroke than if a high pressure fuel system is used. Further, while notshown, the fuel tanks may have a pressure transducer providing a signalto controller 12. It will be appreciated that, in an alternateembodiment, injector 166 may be a port injector providing fuel into theintake port upstream of cylinder 14.

It will also be appreciated that while the depicted embodimentillustrates the engine being operated by injecting fuel via a singledirect injector; in alternate embodiments, the engine may be operated byusing two or more injectors (for example, a direct injector and a portinjector per cylinder, or two direct injectors/two port injectors percylinder, etc.) and varying a relative amount of injection into thecylinder from each injector.

Fuel may be delivered by the injector to the cylinder during a singlecycle of the cylinder. Further, the distribution and/or relative amountof fuel delivered from the injector may vary with operating conditions.Furthermore, for a single combustion event, multiple injections of thedelivered fuel may be performed per cycle. The multiple injections maybe performed during the compression stroke, intake stroke, or anyappropriate combination thereof. Also, fuel may be injected during thecycle to adjust the air-to-injected fuel ratio (AFR) of the combustion.For example, fuel may be injected to provide a stoichiometric AFR. AnAFR sensor may be included to provide an estimate of the in-cylinderAFR. In one example, the AFR sensor may be an exhaust gas sensor, suchas EGO sensor 128. By measuring an amount of residual oxygen (for leanmixtures) or unburned hydrocarbons (for rich mixtures) in the exhaustgas, the sensor may determine the AFR. As such, the AFR may be providedas a Lambda (λ) value, that is, as a ratio of actual AFR tostoichiometry for a given mixture. Thus, a Lambda of 1.0 indicates astoichiometric mixture, richer than stoichiometry mixtures may have alambda value less than 1.0, and leaner than stoichiometry mixtures mayhave a lambda value greater than 1.

As described above, FIG. 2 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel tanks in fuel system 8 may hold fuel with different fuel qualities,such as different fuel compositions. These differences may includedifferent alcohol content, different octane, different heat ofvaporizations, different fuel blends, and/or combinations thereof etc.

Engine 10 may further include a knock sensor 90 coupled to each cylinder14 for identifying abnormal cylinder combustion events. In alternateembodiments, one or more knock sensors 90 may be coupled to selectedlocations of the engine block. The knock sensor may be an accelerometeron the cylinder block, or an ionization sensor configured in the sparkplug of each cylinder. The output of the knock sensor may be combinedwith the output of a crankshaft acceleration sensor to indicate anabnormal combustion event in the cylinder. In one example, based on theoutput of knock sensor 90 in one or more defined windows (e.g., crankangle timing windows), abnormal combustion due to one or more of knockand pre-ignition may be identified and differentiated. Further, theabnormal combustion may be accordingly addressed. For example, knock maybe addressed by reducing the compression ratio and/or retarding sparktiming while pre-ignition is addressed by enriching the engine orlimiting an engine load.

Returning to FIG. 2, controller 12 is shown as a microcomputer,including microprocessor unit 106, input/output ports 108, an electronicstorage medium for executable programs and calibration values shown asread only memory chip 110 in this particular example, random accessmemory 112, keep alive memory 114, and a data bus. Controller 12 mayreceive various signals from sensors coupled to engine 10, in additionto those signals previously discussed, including measurement of inductedmass air flow (MAF) from mass air flow sensor 122; engine coolanttemperature (ECT) from temperature sensor 116 coupled to cooling sleeve118; a profile ignition pickup signal (PIP) from Hall effect sensor 120(or other type) coupled to crankshaft 140; throttle position (TP) from athrottle position sensor; absolute manifold pressure signal (MAP) fromsensor 124, cylinder AFR from EGO sensor 128, and abnormal combustionfrom knock sensor 90 and a crankshaft acceleration sensor. Engine speedsignal, RPM, may be generated by controller 12 from signal PIP. Manifoldpressure signal MAP from a manifold pressure sensor may be used toprovide an indication of vacuum, or pressure, in the intake manifold.The controller 12 receives signals from the various sensors of FIGS. 1-2and employs the various actuators of FIGS. 1-2 to adjust engineoperation based on the received signals and instructions stored on amemory of the controller. For example, adjusting the compression ratioof the engine may include the controller sending a signal to the VCRactuator which actuates the VCR mechanism to mechanically move thepiston closer to or further from the cylinder head, to thereby change avolume of the combustion chamber. As another example, based on signalsfrom the controller, a speed ratio of the transmission of FIG. 1 may bevaried to alter an engine speed-load profile at a given power output.

Non-transitory storage medium read-only memory 110 can be programmedwith computer readable data representing instructions executable byprocessor 106 for performing the methods described below as well asother variants that are anticipated but not specifically listed.

In this way the systems of FIGS. 1-2 provides for a vehicle systemcomprising: an engine with a cylinder; a VCR mechanism coupled to apiston of the cylinder for varying a compression ratio of the engine viamechanical alteration of a piston position within the cylinder; acontinuously variable transmission (CVT) coupling the engine to vehiclewheels, the CVT having a plurality of speed ratios; and a controller.The controller may be configured with computer readable instructionsstored on non-transitory memory for: estimating a first fuel economyassociated with maintaining a first compression ratio to a second fueleconomy associated with transitioning to a second compression ratio; ifthe second fuel economy is higher, predicting a fuel penalty associatedwith operating with a modified engine speed-load profile at the secondcompression ratio; and if the fuel penalty adjusted second fuel economyis higher than the first fuel economy, actuating the VCR mechanism totransition to the second compression ratio while selecting one of theplurality of speed ratios to provide the modified engine speed-loadprofile. Additionally or optionally, the controller may include furtherinstructions for: if the first fuel economy is higher than the fuelpenalty adjusted second fuel economy, maintaining a position of the VCRmechanism to maintain engine operation in the first compression ratio.Additionally or optionally, the modified engine speed-load profile atthe second compression ratio may be a first modified engine speed-loadprofile based on an engine knock limit in the second compression ratio,wherein the controller includes further instructions for: whilemaintaining engine operation in the first compression ratio, predictingthe fuel penalty associated with operating with a second modified enginespeed-load profile at the first compression ratio, the second modifiedengine speed-load profile based on an engine friction loss in the firstcompression ratio; if the fuel penalty is smaller, operating with thesecond modified engine speed-load profile at the first compressionratio; and if the fuel penalty is larger, maintaining a default enginespeed-load profile at the first compression ratio. Additionally oroptionally, the selecting may include selecting a first lower speedratio when the modified engine speed-load profile includes a higherengine speed and a lower engine load, and selecting a second higherratio when the modified engine speed-load profile includes a lowerengine speed and a higher engine load.

Now turning to FIG. 3, an example routine 300 is described forcoordinating adjustments to a compression ratio of an engine configuredwith a VCR mechanism with adjustments to a speed ratio of a continuouslyvariable transmission (CVT). In this way, a powertrain profile of theengine (including engine speed and load) can be adjusted whilemaintaining a demanded power output of the engine and while synergizingfuel economy benefits from each of the VCR adjustment and the CVTadjustment. The method enables improvements in fuel economy despitefrequent changes in driver power demand. Instructions for carrying outmethod 300 as well the other methods included herein may be executed bya controller based on instructions stored on a memory of the controllerand in conjunction with signals received from sensors of the enginesystem, such as the sensors described above with reference to FIGS. 1-2.The controller may employ engine actuators of the engine system toadjust engine operation, according to the methods described below.

At 302, the routine includes estimating and/or measuring engineoperating conditions. These may include, for example, driver powerdemand (such as based on output of a pedal position sensor coupled to anoperator pedal), ambient temperature, pressure and humidity, enginetemperature, fuel level in a fuel tank, fuel octane of availablefuel(s), manifold pressure (MAP), manifold air flow (MAF), catalysttemperature, intake temperature, boost level, etc.

At 304, the method includes, for the given driver power demand,comparing the fuel efficiency at each compression ratio of the engine.In one example, the engine is configured with a VCR mechanism thatmechanically alters the engine compression ratio between a first, lowerand a second, higher CR. In alternate examples, still more compressionratios may be possible. In one example, the controller may compare thefuel efficiency at the two compression ratios by comparing the brakespecific fuel consumption (BSFC) of the engine at each compressionratio. The BSFC of the engine at each compression ratio may be stored intables, maps, and/or equations as a function of operating conditionssuch as RPM, torque, temperature, humidity, inferred fuel octane, etc.

In one example, the engine may be calibrated at each compression ratioto map out islands of constant BSFC. FIG. 6 depicts example BSFC mapswith BSFC islands for an engine at different compression ratios. Inparticular, map 600 depicts BSFC islands at a first, lower compressionratio (e.g., 10.0) and map 620 depicts BSFC islands at a second, highercompression ratio (e.g., 11.9). The maps depict engine speed (in RPM)along the x-axis and engine load or torque or BMEP (in bar) along they-axis. BSFC islands are then plotted based on engine speed relative toload (in g/kW-hr). As such, engine efficiency may be determined as theinverse of BSFC. Thus for any set of BSFC islands, the innermost islandwith the smallest area (island 602 and 622 for maps 600 and 620,respectively) represents an engine operating region with the highestefficiency, and therefore the lowest fuel consumption. In addition, BSFCfor the engine remains constant over a given island.

As can be seen, relative to islands 602, 622, engine efficiency drops asengine speed decreases. This is due to hot gases in the cylinder losingheat to cylinder walls, the losses being more pronounced at lower enginespeeds. Engine efficiency also drops as engine speed increases relativeto islands 602, 622. This is due to increased frictional losses athigher engine speeds. Engine efficiency also drops as torque increasesrelative to islands 602, 622 even though friction accounts for a largerportion of useful engine work in this region. The drop in engineefficiency is due to the need to retard spark to address knock. Finally,engine efficiency drops as torque decreases relative to islands 602, 622due to fixed overheads incurred in the operating of engine componentssuch as oil pumps and water pumps. The pumping work and friction losses(e.g., due to mechanical friction) increase relative to the amount ofwork done, reducing efficiency.

In addition, as the engine CR increases, the size and position of theislands change. In particular, the island of best efficiency for ahigher compression ratio (622) may move to a relatively higher enginespeed and higher engine torque as compared to the island of bestefficiency for a lower compression ratio (602). Also, the island of bestefficiency for the lower compression ratio may encompass a smaller areaspread uniformly over a range of engine speeds and torques (that is,essentially circular in shape) while the island of best efficiency forthe higher compression ratio may encompass a larger area spread over awider range of engine speeds as compared to the range of engine torques(that is, essentially horizontally oval in shape).

At 306, the method includes determining if the fuel efficiency of theengine improves by more than a threshold amount by changing thecompression ratio from the current compression ratio the engine is in toanother compression ratio. For example, the engine may be currentlyoperating at a first, lower compression ratio and in response to achange in the driver demand, it may be determined if the fuel efficiencyof the engine improves by more than a threshold amount by transitioningto a second, higher compression ratio. In another example, the enginemay be currently operating at a second, higher compression ratio and inresponse to the change in the driver demand, it may be determined if thefuel efficiency of the engine improves by more than a threshold amountby transitioning to a first, lower compression ratio. As such, thecontroller may choose the compression ratio providing the lower BFSC asthe more fuel efficient compression ratio. In one example, thecontroller may use the fuel island maps for each compression ratio topre-determine a line of optimal efficiency (calibrated as a function),as elaborated with reference to the example of FIG. 4.

If the fuel efficiency of the engine does not improve by more than thethreshold amount, then at 318, the method includes maintaining thecurrent compression ratio of the engine. Herein, the VCR mechanism (andthereby the piston's TDC position) is maintained in its currentposition. In other words, in response to a lower than thresholdimprovement in fuel efficiency, the current CR is maintained.Optionally, CVT adjustments may be used to adjust the engine speed-loadprofile in the current CR to achieve additional fuel economy benefits.For example, if the current CR is a lower CR, the engine speed may belowered while the engine load is raised to reduce friction losses at lowloads while in the current CR and while maintaining a demanded powerlevel of the engine.

If the fuel efficiency of the engine improves by more than the thresholdamount (based on the fuel efficiency comparison at 304), then at 308,the method includes predicting the engine speed speed and load after theCR transition to the modified CR. In particular, to maintain the poweroutput responsive to the driver demand, the change in CR may result in adifferent engine speed-load profile. For example, for a given driverdemand, the engine may provide the same power output by operating with alower engine speed and higher engine load at the lower compression ratioor with a higher engine speed and lower engine load at the highercompression ratio.

At 310, it may be determined if any engine operating limitations areexpected at the predicted engine speed-load for the more fuel efficientcompression ratio. These may include, for example, knock limitations, orfrictional losses. For example, it may be determined if knock is likelyto occur at the predicted engine speed-load.

If knock is not expected at the predicted engine speed-load, then at312, the method includes transitioning the engine to the modifiedcompression ratio with the higher fuel efficiency via adjustments to theVCR mechanism. Herein the controller may transition to the compressionratio providing the lower BFSC. This includes the controller sending asignal to a VCR actuator coupled a VCR mechanism that mechanicallyalters the piston TDC position within the cylinder. For example thesignal to the VCR actuator may actuate the VCR mechanism to a positionwhere the piston position within the cylinder corresponds to theselected compression ratio. In one example, the VCR mechanism is apiston position changing mechanism. In another example, the VCRmechanism is a cylinder head volume changing mechanism.

For example, the engine may be configured with a VCR mechanism thatmechanically alters the compression ratio of the engine between a first,lower compression ratio and a second, higher compression ratio. When thelower CR is the more fuel efficient CR, the controller may transition tothe engine to the lower CR (from the higher CR), the engine operating atthe lower compression ratio including the controller sending a signal tothe VCR actuator to move the VCR mechanism. For example, the compressionratio may be reduced by reducing the piston position within a cylindervia one of an elliptical crankshaft rotation and an eccentric coupled toa piston pin and a variable height piston crown and a variable lengthconnecting rod and an unconventional cranktrain linkage. In anotherexample, when the higher CR is the more fuel efficient CR, thecontroller may transition to the engine to the higher CR (from the lowerCR), the engine operating at the higher compression ratio including thecontroller sending a signal to the VCR actuator to move the VCRmechanism. For example, the compression ratio may be increased byincreasing the piston position within a cylinder via one of anelliptical crankshaft rotation and an eccentric coupled to a piston pinand a variable height piston crown and a variable length connecting rodand an unconventional cranktrain linkage.

The routine may then move to 320 wherein the controller adjusts the CVTto provide the engine speed-load profile that is optimal for theselected CR. For example, the CVT may be adjusted to a lower speed ratioresponsive to a transition to the lower CR, thereby lowering the enginespeed. As another example, the CVT may be adjusted to a higher speedratio responsive to a transition to the higher CR, thereby raising theengine speed.

In addition to actuating the VCR mechanism and the CVT, the controllermay also actuate one or more of the engine intake throttle, intakeand/or exhaust cams, valve lift, boost pressure, and spark timing todeliver the optimal load (torque) for the selected compression ratio.

Returning to 310, if knock is expected at the predicted enginespeed-load, then at 314, the method includes predicting a fuel penaltyassociated with a knock mitigating adjustment. For example, it may bedetermined if the engine speed-load profile can be modified (viaadjustments to a speed ratio of the CVT) to reduce the knock. Adjustingto the adjusted/modified engine speed-load profile may include selectinga speed ratio of the CVT that matches the adjusted engine speed-loadprofile. This may include selecting a speed ratio that raises the enginespeed while lowering the engine load while at the higher compressionratio to maintain the power output of the engine while reducing knock.In one example, the engine speed may be increased while the engine loadis decreased as engine operation in the modified CR approaches the knocklimit. As such, the inventors have recognized that the engine speed-loadprofile may be varied while maintaining the engine power output viaadjustments to a speed ratio of the CVT. Then, the fuel efficiency ofthe engine in the new engine speed-load profile and the selected CR maybe calculated (in the present example, the fuel efficiency at the higherspeed and lower load of the higher CR). In one example, the controllermay refer a map, such as the example map of FIG. 4 (elaborated below) todetermine if the change in engine speed-load results in transition ofthe engine from a first line (or island) of best fuel efficiency to asecond, different line (or island) of best fuel efficiency, the secondline having a lower fuel efficiency as compared to the first line.Herein, a fuel penalty may be estimated based on a drop in the fuelefficiency (e.g., based on a difference between the fuel efficiency atthe first line relative to the second line). With reference to theabove-described example, a first fuel efficiency of the engine at thehigher CR with the default lower engine speed and higher engine load maybe compared to a second fuel efficiency of the engine at the higher CRwith the CVT-adjusted higher engine speed and lower engine load. In analternate example, an amount of spark retard required to mitigate theknock may be determined and the corresponding fuel penalty may becomputed.

At 316, the predicted fuel penalty associated with knock (Knk_fuelpenalty) may be compared to the fuel economy change associated with thetransition to the selected CR (CR_fuel economy). In other words, a fueleconomy associated with operating the engine at the current CR iscompared to the fuel economy change associated with operating the engineat the modified CR with the modified engine speed-load profile. Herein,the engine may be operated at the current CR with either a defaultengine speed-load profile for the current CR or a modified speed-loadprofile modified based on engine limitations at the current CR (whichmay be the same as or different from the engine limitations at themodified CR). Thus, the modified speed-load profile at the current CRmay be different from the modified speed-load profile at the modifiedCR. For example, the modified speed-load profile at the current CR maybe adjusted for friction losses while the modified speed-load profile atthe modified CR may be adjusted for knock limitations. However, a powerlevel of the engine is maintained at each of engine operation in thecurrent CR (with default or modified speed-load profile) and the engineoperation in the modified CR with the modified/adjusted speed-loadprofile.

If the fuel penalty is lower than the fuel economy change due to CR,that is, the net result even with the change in engine speed-loadprofile and the CR transition is a fuel benefit, then the method movesto 312 to transition the engine to the compression ratio with the higherfuel efficiency via adjustments to the VCR mechanism. This includes thecontroller sending a signal to a VCR actuator coupled a VCR mechanism tomove the VCR mechanism.

In addition, at 320, the controller may adjust the CVT to provide theengine speed-load profile that is optimal for the selected CR and foraddressing the given knock limitations. For example, the engine may betransitioned to the higher CR while the CVT is adjusted to a speed ratiothat raises the engine speed and lowers the engine load whilemaintaining the same engine powertrain output.

If the fuel penalty is higher than the fuel economy change due to CR,that is, the net result even with the change in engine speed-loadprofile and the CR transition is a fuel loss, then the method moves to318 to maintain the engine in the current compression ratio. Thus, eventhough the other CR may be nominally more fuel efficient for the givendriver demand, the controller may maintain the engine in the current CRin view of fuel inefficient limitations that may be experienced whenoperating at the other CR. Maintaining the current CR includes thecontroller sending a signal to the VCR actuator coupled the VCRmechanism to maintain the position of the VCR mechanism. In addition, at320, the controller may adjust the CVT to provide the engine speed-loadprofile that is optimal for the selected CR. This may includemaintaining a default engine speed-load profile for the current CR whilemaintaining the current CR. Alternatively, this may include adjustingthe engine speed-load profile for the current CR via adjustments to theCVT speed ratio while maintaining the current CR.

It will be appreciated that while the above method discusses predictingknock limitations at the predicted engine speed-load at 310, andpredicting a knock mitigating fuel penalty at 314, this is not meant tobe limiting. In an alternate example, the controller may predictfrictional losses at the predicted engine speed-load and then predict afriction mitigating fuel penalty. For example, when operating with alower CR at lower loads, friction may be traded for knock constraints.Thus, it may be more fuel efficient to transition the engine to thelower CR while the CVT is adjusted to a speed ratio that lowers theengine speed and raises the engine load while maintaining the sameengine power output.

In one example, as elaborated with reference to FIG. 4, the data fromfuel island data maps for each compression ratio may be reduced to twobest efficiency lines that the controller can more quickly interpolatebetween in real-time. Otherwise, the controller would have to run anoptimization on each fuel map, and then try to further optimize a pointin-between the two compression ratio states. In the present approach,the controller may for use fuel island maps for compression ratio topre-determine a line of optimal efficiency. Then, for the current powerdemand, the controller may look up lines of optimal efficiency for acurrent operating power and evaluate the two curves to determine optimalefficiency. If the CR is variable (e.g., if the CR changes 40% from afirst CR, CR1, to a second CR, CR2), the controller may interpolatelinearly between the line of optimal efficiency for the first CR and thesecond CR. Although the line may not be exactly linear, the changes maybe small enough that a linear approximation may be a reasonablereal-time approximation.

Turning now to FIG. 4, an example map 400 is shown for comparing fuelefficiencies associated with different CRs for a given driver demandedpower output, as well as for comparing fuel efficiencies associated withdifferent engine speed-load profiles for a given CR. In one example, themap of FIG. 4 may be generated during engine calibration and stored inthe engine controller's memory. The controller may then reference themap during engine operation to determine whether to maintain a currentCR or transition to an alternate CR responsive to a change in driverdemand.

Map 400 depicts a first line of best efficiency versus power at a highercompression ratio, herein also referred to as a higher CRoptimum-efficiency load limit 404 (depicted as a solid line). Map 400also depicts a second line of best efficiency versus power at a lowercompression ratio, herein also referred to as a lower CRoptimum-efficiency load limit 406 (depicted as a dashed line). The plotsare shown with engine speed along the x-axis and engine load or torquealong the y-axis. An example BSFC island (herein oval) of best fuelefficiency for the higher compression ratio is overlaid at dotted line408, while a corresponding island for the lower compression ratio isoverlaid at dotted line 409. It will be appreciated that islands 408 and409 represent the innermost island of lowest fuel consumption and thatfuel islands outer to this island are not shown herein for reasons ofclarity. As such, the exact positions of the ovals of constantefficiency will change depending on current knock limits, which varywith fuel octane, temperature, humidity, and of course compressionratio. The maximum torque of the engine at a given engine speed is shownby curve 402. Lines of constant power output corresponding to 10 kW-50kW are depicted at power lines 450-490, respectively.

A first CR and CVT adjustment is shown with reference to operatingpoints 410-416. Based on a current driver demand, the engine may be atoperating point 410 on the engine speed-load map. In particular, basedon the engine load corresponding to a position on (or just below) higherCR optimum-efficiency load limit 404, and a power demand of 10 kW, theengine may be operating at operating point 410 with the highercompression ratio and with an engine speed/load along power line 450.The engine speed-load at the current CR may be selected based on BSFCisland 408.

If there is an increase in driver demand to 20 kW (such as due to anoperator pedal tip-in while the engine is in the higher compressionratio), the engine may transition to operate along power line 460 anddetermine whether to stay in the higher CR or transition to the lower CRbased on changes in fuel efficiency. As a first option, the engine couldbe moved to operating point 412 along power line 460. Herein, the driverdemand is provided while maintaining the current higher CR. As a secondoption, the engine could be moved to operating point 414 along powerline 460 where the same power output is provided while transitioning tothe lower CR. As such, for the given driver demand, a higher fuelefficiency is provided at the lower CR, due to the engine operating onan island of higher fuel efficiency at point 414. However, thecontroller may further determine that operating point 414 is associatedwith a limitation (e.g., a knock limitation) which can be addressed bymoving, as a third option, to operating point 416 where engine load isincreased and engine speed is decreased while staying on power line 460.The engine speed-load adjustment may be performed via adjustments to aspeed ratio for the CVT. Moving to operating point 416 would result in adrop in fuel economy (that is, incur a fuel penalty) relative to stayingat operating point 414. However, the fuel penalty associated with thetransition from operating point 414 to operating point 416 is smallerthan the fuel improvement associated with the transition from operatingpoint 412 to operating point 414. Consequently, in response to theincrease in driver demand, it is more fuel efficient to move fromoperating point 410 to 416 by transitioning to the lower compressionratio and decreasing the engine speed while lowering the engine load inthe higher CR.

It will be appreciated that if the engine speed-load adjustment requiredto address the limitation moved the engine, as a fourth option, tooperating point 418 (where the engine load is further increased andengine speed is further decreased while staying on power line 460), themove would incur a larger fuel penalty. In that case, the fuel penaltyassociated with the transition from operating point 414 to operatingpoint 418 would be predicted to be larger than the fuel improvementassociated with the transition from operating point 412 to operatingpoint 414. Consequently, in response to the increase in driver demand,it would be more fuel efficient to move from operating point 410 to 414by maintaining the higher compression ratio.

A second CR and CVT adjustment is shown with reference to operatingpoints 420-424. Based on a current driver demand, the engine may be atoperating point 420 on the engine speed-load map. In particular, basedon the engine load corresponding to a position on (or just below) higherCR optimum-efficiency load limit 404, and a power demand of 40 kW, theengine may be operating at operating point 420 with the highercompression ratio (that is, with the VCR mechanism actuated to aposition where the distance between the piston and the cylinder head ishigher) and with an engine speed/load along power line 480. The enginespeed-load at the current CR may be selected based on BSFC island 409.

If there is an increase in driver demand to 50 kW (such as due to anoperator pedal tip-in while the engine is in the higher compressionratio), the engine may transition to operate along power line 490 anddetermine whether to stay in the higher CR or transition to the lower CRbased on changes in fuel efficiency. As a first option, the engine couldbe moved to operating point 422 along power line 490. Herein, the driverdemand is provided while maintaining the current higher CR. As a secondoption, the engine could be moved to operating point 424 along powerline 490 where the same power output is provided while transitioning tothe lower CR. As such, for the given driver demand, a higher fuelefficiency is provided at the higher CR, as determined based on acomparison of their BSFC islands. Consequently, in response to theincrease in driver demand, it is more fuel efficient to move fromoperating point 420 to 424 by maintaining the higher compression ratio.Additionally, further fuel economy benefits can be achieved via CVTadjustments while staying in the higher CR. Specifically, a speed ratioof the CVT can be adjusted to move the engine to operating point 426along power line 490 where the same power output is provided whilemaintaining the higher CR by decreasing the engine load while increasingthe engine speed.

In this way, for a given driver demand, an engine controller mayestimate a first fuel economy associated with maintaining a firstcompression ratio to a second fuel economy associated with transitioningto a second compression ratio while operating with a knock-adjustedengine speed-load profile. If the second fuel economy is higher than thefirst fuel economy, the controller may determine that is more fuelefficient to transition, and the controller may transition the engine tothe second compression ratio via mechanical adjustments to a pistonposition (such as via the VCR mechanism). In addition, the controllermay transition the engine to the knock-adjusted engine speed-loadprofile via adjustments to a speed ratio of the CVT. Herein,transitioning to the knock-adjusted engine speed-load profile includestransitioning from a default engine speed-load profile of the secondcompression ratio. In one example, the knock-adjusted engine speed-loadprofile includes a higher engine speed and a lower engine load ascompared to the default engine speed-load profile for a given powerlevel at the given compression ratio. In addition, an engine poweroutput during engine operation at the first compression ratio is same asthe engine power output during engine operation at the secondcompression ratio with the knock-adjusted engine speed-load profile. Incomparison, if the second fuel economy is smaller than the first fueleconomy, the controller may determine that is not fuel efficient totransition, and the controller may maintain the first compression ratio.Additionally or optionally, the controller may transition to afriction-adjusted engine speed-load profile via adjustments to the speedratio of the CVT while in the first compression ratio if thefriction-adjusted engine speed-load profile provides even more fueleconomy benefits (than staying in the first compression ratio with thedefault engine speed-load profile). In one example, where the secondcompression ratio is higher than the first compression ratio, theknock-adjusted engine speed-load profile at the second compression ratioincludes a higher than default engine speed and a lower than defaultengine load, while the friction-adjusted engine speed-load profile atthe first compression ratio includes a lower than default engine speedand a higher than default engine load.

Now turning to FIG. 5, map 500 depicts example CVT and VCR adjustmentsthat may be integrated to provide synergistic fuel economy benefits. Map500 depicts changes to an engine speed at plot 502, changes to an engineload at plot 504, changes to an engine power output at plot 506, changesto an engine compression ratio (CR) at plot 508, and a knock sensoroutput at plot 510. It will be appreciated that as used herein, theengine power is determined as a product of engine speed and engine load(or torque). In addition, the engine speed-load adjustments are achievedvia adjustments a speed ratio of a CVT coupled between the engine and anoutput shaft. In the present example, the CR is adjustable between afirst and a second value, although in alternate examples, additional CRsmay be possible and/or the CR may be adjustable to any CR between thefirst and second values.

Prior to t1, the engine may be operating to provide a power output thatis delivered via the depicted engine speed-load profile and with theengine in the higher CR. At t1, in response to an increase in driverdemand, the power output of the engine may be increased. Herein, thepower output is increased by transitioning to the lower CR due to thehigher CR being more fuel efficient than the higher CR. In addition,further fuel economy benefits are achieved by adjusting the enginespeed-load profile in the lower CR via CVT adjustments so that the sameengine power is provided using a higher than default engine speed and alower than default engine load. The default engine speed and load (forthe given CR) are depicted here as dashed lines. In particular, if theengine were maintained in the higher CR and operated with the defaultengine speed-load, the engine would have been knock limited, asindicated by predicted knock sensor output 512 (dashed segment)exceeding the knock threshold (Knk_Thr). Herein, by transitioning to thehigher engine speed and lower engine load via CVT adjustments whiletransitioning to the lower CR via VCR adjustments, knock at higher loadsis addressed while improving the overall engine fuel economy, andwithout compromising engine power output.

The engine may operate with the higher than default engine speed andlower than default engine load at the lower CR for a duration until t2.At t2, in response to a drop in driver demand, the engine may bemaintained in the lower CR while resuming the default engine speed anddefault engine load due to the engine not being knock limited any more.This operation may be maintained until t3.

At t3, in response to a decrease in driver demand, the power output ofthe engine may be decreased. Herein, the power output is decreased bytransitioning to the higher CR due to the higher CR being more fuelefficient than the lower CR. In addition, further fuel economy benefitsare achieved by adjusting the engine speed-load profile in the higher CRvia CVT adjustments so that the same engine power is provided using alower than default engine speed and a higher than default engine load.The default engine speed and load (for the given CR) are depicted hereas dashed lines. In particular, if the engine were transitioned to thelower CR and operated with the default engine speed-load, the enginecould have been friction limited. Herein, by transitioning to the lowerengine speed and higher engine load via CVT adjustments whiletransitioning to the higher CR via VCR adjustments, friction losses atlower loads are reduced, while improving the overall engine fueleconomy, and without compromising engine power output.

In this way, fuel efficiency of an engine can be improved by integratingVCR technology with CVT technology. By leveraging the different enginespeed-load combinations achievable for a given engine power output viaCVT adjustments, an engine controller can more accurately address enginelimitations, such as knock limitations associated with a compressionratio transition. As such, this allows the fuel costs associated with acompression ratio transition to be more accurately determined, reducingthe frequency of fuel inefficient compression ratio switches responsiveto frequent changes in operator or wheel torque demand. Overall, fueleconomy of an engine can be enhanced.

One example method for an engine coupled with a continuously variabletransmission (CVT), comprises: for a power level, comparing engineefficiency at a current compression ratio to engine efficiency at amodified compression ratio with an adjusted engine speed-load; and inresponse to a higher than threshold improvement in the engine efficiencyat the modified compression ratio with the adjusted engine speed-load,transitioning to the modified compression ratio and adjusting to theadjusted engine speed-load. In the preceding example, additionally oroptionally, the method further comprises, in response a lower thanthreshold improvement in the engine efficiency, maintaining the currentcompression ratio. In any or all of the preceding examples, additionallyor optionally, the method further comprises adjusting the enginespeed-load while maintaining the current compression ratio, the adjustedengine speed-load with the current compression ratio different from theadjusted engine speed-load with the modified compression ratio. In anyor all of the preceding examples, additionally or optionally, theadjusted engine speed-load with the modified compression ratio is basedon a knock limit of the engine at the modified compression ratio. In anyor all of the preceding examples, additionally or optionally, adjustingto the adjusted engine speed-load includes increasing the engine speedwhile decreasing the engine load to maintain the power level as engineoperation at the modified compression ratio approaches the knock limit.In any or all of the preceding examples, additionally or optionally,adjusting to the adjusted engine speed-load includes selecting a CVTspeed ratio matching the adjusted engine speed-load. In any or all ofthe preceding examples, additionally or optionally, the power level ismaintained at each of engine operation in the current compression ratioand engine operation in the modified compression ratio with the adjustedengine speed-load. In any or all of the preceding examples, additionallyor optionally, the power level is a powertrain output of the enginedetermined as a product of engine load and engine speed. In any or allof the preceding examples, additionally or optionally, transitioning tothe modified compression ratio includes actuating a variable compressionratio mechanism to mechanically alter a piston position within acylinder of the engine. In any or all of the preceding examples,additionally or optionally, the variable compression ratio mechanism isa piston position changing mechanism including one of an ellipticalcrankshaft and an eccentric coupled to a piston pin. In any or all ofthe preceding examples, additionally or optionally, the variablecompression ratio mechanism is a cylinder head volume changingmechanism.

Another example method for an engine coupled with a continuouslyvariable transmission (CVT), comprises: for a driver demand, estimatinga first fuel economy associated with maintaining a first compressionratio to a second fuel economy associated with transitioning to a secondcompression ratio while operating with a knock-adjusted enginespeed-load profile; and in response to the second fuel economy beinghigher than the first fuel economy, transitioning to the secondcompression ratio via mechanical adjustments to a piston position andtransitioning to the knock-adjusted engine speed-load profile viaadjustments to a speed ratio of the CVT. In the preceding example,additionally or optionally, transitioning to the knock-adjusted enginespeed-load profile includes transitioning from a default enginespeed-load profile of the second compression ratio. In any or all of thepreceding examples, additionally or optionally, an engine power outputduring engine operation at the first compression ratio is same as theengine power output during engine operation at the second compressionratio with the knock-adjusted engine speed-load profile. In any or allof the preceding examples, additionally or optionally, the methodfurther comprises, in response to the second fuel economy being smallerthan the first fuel economy, maintaining the first compression ratio andoptionally transitioning to a friction-adjusted engine speed-loadprofile via adjustments to the speed ratio of the CVT. In any or all ofthe preceding examples, additionally or optionally, the secondcompression ratio is higher than the first compression ratio, whereinthe knock-adjusted engine speed-load profile includes a higher thandefault engine speed and a lower than default engine load, and whereinthe friction-adjusted engine speed-load profile includes a lower thandefault engine speed and a higher than default engine load.

Another example vehicle system comprises: an engine with a cylinder; aVCR mechanism coupled to a piston of the cylinder for varying acompression ratio of the engine via mechanical alteration of a pistonposition within the cylinder; a continuously variable transmission (CVT)coupling the engine to vehicle wheels, the CVT having a plurality ofspeed ratios; and a controller. The controller may be configured withcomputer readable instructions stored on non-transitory memory for:estimating a first fuel economy associated with maintaining a firstcompression ratio to a second fuel economy associated with transitioningto a second compression ratio; if the second fuel economy is higher,predicting a fuel penalty associated with operating with a modifiedengine speed-load profile at the second compression ratio; and if thefuel penalty adjusted second fuel economy is higher than the first fueleconomy, actuating the VCR mechanism to transition to the secondcompression ratio while selecting one of the plurality of speed ratiosto provide the modified engine speed-load profile. In the precedingexample, additionally or optionally, the controller includes furtherinstructions for: if the first fuel economy is higher than the fuelpenalty adjusted second fuel economy, maintaining a position of the VCRmechanism to maintain engine operation in the first compression ratio.In any or all of the preceding examples, additionally or optionally, themodified engine speed-load profile at the second compression ratio is afirst modified engine speed-load profile based on an engine knock limitin the second compression ratio, wherein the controller includes furtherinstructions for: while maintaining engine operation in the firstcompression ratio, predicting the fuel penalty associated with operatingwith a second modified engine speed-load profile at the firstcompression ratio, the second modified engine speed-load profile basedon an engine friction loss in the first compression ratio; if the fuelpenalty is smaller, operating with the second modified engine speed-loadprofile at the first compression ratio; and if the fuel penalty islarger, maintaining a default engine speed-load profile at the firstcompression ratio. In any or all of the preceding examples, additionallyor optionally, the selecting includes selecting a first lower speedratio when the modified engine speed-load profile includes a higherengine speed and a lower engine load, and selecting a second higherratio when the modified engine speed-load profile includes a lowerengine speed and a higher engine load.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A method for an engine coupled with acontinuously variable transmission (CVT), comprising: for a driverdemand, estimating a first fuel economy associated with maintaining afirst compression ratio to a second fuel economy associated withtransitioning to a second compression ratio while operating with aknock-adjusted and friction-adjusted engine speed-load profile; and inresponse to the second fuel economy being higher than the first fueleconomy, transitioning to the second compression ratio via mechanicaladjustments to a piston position and transitioning to the knock-adjustedand friction-adjusted engine speed-load profile via adjustments to aspeed ratio of the CVT.
 2. The method of claim 1, wherein transitioningto the knock-adjusted and friction-adjusted engine speed-load profileincludes transitioning from a default engine speed-load profile of thefirst compression ratio, and wherein an engine power output duringengine operation at the first compression ratio is the same as theengine power output during engine operation at the second compressionratio with the knock-adjusted and friction-adjusted engine speed-loadprofile.
 3. The method of claim 1, further comprising, in response tothe second fuel economy being smaller than the first fuel economy,maintaining the first compression ratio and transitioning to theknock-adjusted and friction-adjusted engine speed-load profile viaadjustments to the speed ratio of the CVT.
 4. The method of claim 3,wherein the second compression ratio is higher than the firstcompression ratio, and wherein the knock-adjusted and friction-adjustedengine speed-load profile includes a higher than default engine speedand a lower than default engine load.
 5. The method of claim 4, whereinthe second compression ratio is lower than the first compression ratio,and wherein the knock-adjusted and friction-adjusted engine speed-loadprofile includes a lower than default engine speed and a higher thandefault engine load.
 6. A vehicle system, comprising: an engine with acylinder; a VCR mechanism coupled to a piston of the cylinder forvarying a compression ratio of the engine via mechanical alteration of apiston position within the cylinder; a continuously variabletransmission (CVT) coupling the engine to vehicle wheels, the CVT havinga plurality of speed ratios; and a controller with computer readableinstructions stored in non-transitory memory for: estimating a firstfuel economy associated with maintaining a first compression ratio to asecond fuel economy associated with transitioning to a secondcompression ratio; if the second fuel economy is higher, predicting afuel penalty adjusted second fuel economy associated with operating witha modified engine speed-load profile at the second compression ratio;and if the fuel penalty adjusted second fuel economy is higher than thefirst fuel economy, actuating the VCR mechanism to transition to thesecond compression ratio while selecting one of the plurality of speedratios to provide the modified engine speed-load profile.
 7. The systemof claim 6, wherein the controller includes further instructions for: ifthe first fuel economy is higher than the fuel penalty adjusted secondfuel economy, maintaining a position of the VCR mechanism to maintainengine operation in the first compression ratio.
 8. The system of claim6, wherein the modified engine speed-load profile at the secondcompression ratio is a first modified engine speed-load profile based onan engine knock limit and engine friction in the second compressionratio, wherein the controller includes further instructions for: whilemaintaining engine operation in the first compression ratio, predictinga fuel penalty associated with operating with a second modified enginespeed-load profile at the first compression ratio, the second modifiedengine speed-load profile based on an engine knock limit and enginefriction in the first compression ratio; if the fuel penalty is smaller,operating with the second modified engine speed-load profile at thesecond compression ratio; and if the fuel penalty is larger, maintaininga default engine speed-load profile at the first compression ratio. 9.The system of claim 6, wherein the selecting includes selecting a firstlower speed ratio when the modified engine speed-load profile includes ahigher engine speed and a lower engine load, and selecting a secondhigher ratio when the modified engine speed-load profile includes alower engine speed and a higher engine load.